Service PO Locking

Hi Expert

Can any one tell me how service PO can be restricted based on document total. Let me explain the scenario, if there is service PO of amount 1000, if i made AP Invoice of amount 500 first time and amount 600 second times system allows this (SAP B1 2007B), I need such restriction where system will not allow to make AP invoice more than amount mention in service PO.

How this can be done, this there any way in transaction notification?

Thank you